It is heretofore been known to employ electric motors to open and close portal closing members. The most recognizable example of such portal operators are electric garage door openers. However, the same technique is employed in other apparatuses such as toll gates, railroad crossing gates, and operators for large industrial gates. The common feature in all of these is the use of an electric motor to open and close the portal closing member. This electrical motor is used either for convenience, as in the case of electric garage door openers, or for supplying the relatively great motive power required, as in the case of large industrial gates not suitable for manual operation.
The use of such automatic portal operators introduces a potential safety hazard. Such automatic portal operators do not have the capability of recognizing an obstruction or other hazardous condition as is the case when such a portal was operated by a person. There have been various techniques employed in order to attempt to detect obstructions of the automatic portal operator and to initiate some safety function to reduce or eliminate the hazardous operation. The most common form of safety device is a strip at the leading edge of the portal member, which strip includes a switch which is operated when it strikes an object. Thus, striking an object causes the switch to actuate and enables the controller for the automatic portal operator to initiate a safety operation.
Another technique known in the art is that of U.S. Pat. No. 4,335,339 entitled "Electronic Safety Device" issued to Joseph L. Brickner on June 15, 1982. In accordance with this patent, the amount of current drawn by the electric motor of the automatic portal operator is measured. A sudden increase in the current drawn by the motor, which is indicative of a sudden increase in the motor load, enables triggering of the safety function.
In accordance with the prior art it is heretofore been known to provide one of three types of safety operations. The first type of safety operation is to stop the motor and thereby stop the portal from opening or closing. Another type of safety operation is to reverse the operation of the portal operator and return to the limit opposite that which the portal was being driven. A third, more sophisticated technique, is to stop the portal operator, drive the portal in the reverse direction for a predetermined amount, and then again stop. Each of these three techniques may be advantageously employed for enhancing the safety of the operation of the portal operator, depending upon the nature of the portal and the potential hazards.
The above described safety techniques have limited capability in recognizing hazardous conditions. In the case of a safety strip at the leading edge of the portal, this responds only when the portal is traveling in the closing direction. It is likely that an obstruction be encountered on the trailing edge or on the middle of the portal. Therefore, even in the case of the portal operator closing, it is possible to create a hazardous condition at portions of the portal other than the leading edge where the safety strip is located. In such a case the safety strip would not detect the hazardous condition.
In the case of the sensing of the current of the drive motor, the current is not necessarily indicative of a overload condition of the motor. Firstly, the current may vary widely during normal operation of the motor without the presence of obstructions. In order to achieve a sufficiently low number of false alarms in such a case, the current must be set higher than the highest expected current during any normal operation. In addition, it is possible to get abnormally high current under adverse conditions of electric supply voltage, such as during a brown out in which the voltage is substantially reduced. This leads to false positive operation of the safety function. In addition, it is also possible to get false negative operations in which unusual power supply conditions are coupled with the occurrence of an obstruction or blockage of the portal during operation. Therefore, these techniques cannot be regarded as reliably detecting obstructions in the operation of the automatic portal operator apparatus.
Accordingly it has been a need in the art to provide some method of easily and reliably detecting an unsafe operation of an automatic portal operator, in particular the case in which the automatic portal operator encounters an unexpected obstruction.